Non-structural protein 1 of avian influenza A viruses differentially inhibit NF-κB promoter activation

Background: Influenza virus infection activates NF-kappa B and is a general prerequisite for a productive influenza virus infection. On the other hand, non-structural protein 1 (NS1) suppresses this viral activated NF-kappa B, presumably to prevent expression of NF-kappa B mediated anti-viral response. NS1 proteins of influenza A viruses are divided into two groups, known as allele A and allele B. The possible functional relevance of this NS1 division to viral pathogenicity is lacking. Findings: The ability of NS1 protein from two avian influenza subtypes, H6N8 and H4N6, to inhibit NF-kappa B promoter activation was assessed. Further, efforts were made to characterize the genetic basis of this inhibition. We found that allele A NS1 proteins of H6N8 and H4N6 are significantly better in preventing dsRNA induced NF-kappa B promoter activation compared to allele B of corresponding subtypes, in a species independent manner. Furthermore, the ability to suppress NF-kappa B promoter activation was mapped to the effector domain while the RNA binding domain alone was unable to suppress this activation. Chimeric NS1 proteins containing either RNA binding domain of allele A and effector domain of allele B or vice versa, were equally potent in preventing NF-kappa B promoter activation compared to their wt. NS1 protein of allele A and B from both subtypes expressed efficiently as detected by Western blotting and predominantly localized in the nucleus in both A549 and MiLu cells as shown by in situ PLA. Conclusions: Here, we present another aspect of NS1 protein in inhibiting dsRNA induced NF-kappa B activation in an allele dependent manner. This suggests a possible correlation with the virus's pathogenic potential.